1. Field of the Invention
Embodiments of the present invention relate to semiconductor apparatuses. Specifically, the embodiments of present invention relate to semiconductor apparatuses having a package structure including a base and a case bonded to the base with an adhesive.
2. Related Art
FIG. 3(A) is the top plan view of a conventional semiconductor apparatus showing an example of a package structure. FIG. 3(B) is the cross sectional view along a-a of FIG. 3(A). FIG. 3(C) is the expanded illustration of the part b in FIG. 3(B).
In the semiconductor apparatus shown in FIGS. 3(A) through 3(C), insulator baseboard 11 is mounted on metal base 10 for heat dissipation. Semiconductor chips 12, 13, 14, and 15 are mounted on insulator baseboard 11. Insulator baseboard 11 is bonded to metal base 10 with a solder via a metal foil formed on one surface of insulator baseboard 11. To another metal foil formed on the other surface of insulator baseboard 11, semiconductor chips 12, 13, 14, and 15 are bonded with a solder. Semiconductor chips 12, 13, 14, and 15 may include an insulated-gate bipolar transistor (IGBT) and a free-wheeling diode.
Frame-shaped resin case 16 is put on insulator base board 11 such that resin case 16 surrounds insulator baseboard 11. Resin case 16 is bonded to baseboard 11 with adhesive 17. External terminals are inserted to resin case 16 such that the resin case and the external terminals are formed into a unit. As for the external terminals, external main circuit terminals 18, 19, 20 and 21 and external control terminals 22, 23, 24, and 25 are arranged for semiconductor chips 12 and 14. Semiconductor chips 12 and 14 and external main circuit terminals 18, 19, 20 and 21 are connected electrically to each other by bonding wires 26. Semiconductor chips 12 and 14 and external control terminals 22, 23, 24, and 25 are connected electrically to each other by bonding wires 27.
Resin case 16 is filled with a silicone gel to seal the circuits connected by the wire bondings therein. Resin case 16 is closed with a cover (not shown). Although not illustrated, a bolt for fastening the semiconductor apparatus to a radiator fin is inserted to hole 28 bored through metal base 10 outside resin case 16.
For bonding resin case 16 to metal base 10, a protrusion is formed on the edge of the resin case 16 surface facing metal base 10, the gap caused by the protrusion between resin case 16 and metal base 10 is filled with adhesive 17, and adhesive 17 is cured thermally. This bonding method facilitates obtaining a sufficient bonding strength, since the bonding area is wide. However, it is necessary to coat a large amount of adhesive 17.
To obviate this problem, Japanese Unexamined Patent Application Publication No. 2003-51560 (also referred to herein as “Patent Document 1”) and Japanese Unexamined Patent Application Publication No. 2004-103846 (also referred to herein as “Patent Document 2”) propose a technique as described below. The proposed technique forms a groove in the resin case surface facing the metal base, and fills the groove with an adhesive to suppress the coating amount of the adhesive. The groove extends continuously along all the peripheral sides of the resin case surface. Here, it is necessary to raise the bonding strengths in the vicinity of the portion of the resin case, to which external main circuit terminals are inserted, and in the vicinity of the portion of the resin case, to which external control terminals are inserted. In the portions of the resin case described above (hereinafter referred to as the “terminal-insertion portions”), the bonding wires are bonded by ultrasonic welding. Since the ultrasonic vibration forces are exerted to the terminal-insertion portions, it is required for the terminal-insertion portions to exhibit an especially high bonding strength. Therefore, a plurality of grooves is formed in the terminal-insertion portions to improve the bonding strength thereof.
The adhesive is coated to the groove with a dispenser capable of controlling the coating amount of the adhesive. If the coating amount is insufficient, a sufficient bonding strength is not secured. If peelings or cracks are caused in the adhesive, the silicone gel loaded in the resin case will leak outside. Therefore, the dispenser coats to the groove more than a necessary amount of the adhesive.
If the coating amount of the adhesive to the groove is too large, an excess amount of the adhesive will flow out from the groove to the outside or the inside of the resin case when the resin case is laid on the metal base. If the adhesive flows to the outside of the resin case, the external appearance of the resin case will be impaired. Moreover, if the adhesive which has flowed out sticks to the contact plane of the metal base and the radiator fin and hardens there, the surface height of the metal base will become nonuniform and the cooling efficiency will be impaired. If the adhesive which has flowed to the inside of the resin case contaminates the bonding planes of the bonding wires, the strength of the bonding portion formed by ultrasonic welding will be lowered. Japanese Unexamined Patent Application Publication No. Hei. 7 (1995)-30006 (also referred to herein as “Patent Document 3”) describes a method that provides the resin case with an adhesive escape groove to prevent an excess amount of the adhesive from flowing out.
However, the gap between the resin case plane except the groove and the metal base plane is narrow in the state, in which the resin case is pressed to the metal base and the adhesive is heated and hardened. If the coating amount of the adhesive is excess even slightly, the adhesive which has flowed out expands through the narrow gap widely and flows to the outside and inside of the resin case. Even if an adhesive escape groove is formed in the area, in which many grooves are formed, it will be difficult for the adhesive escape groove to prevent the adhesive from flowing out. It is also difficult to control the coating amount of the adhesive exactly so that the amount of the adhesive flowing out may be small as much as possible.
In view of the foregoing, it would be desirable to obviate the problems described above. It would be also desirable to provide a semiconductor apparatus which has a bonding structure that facilitates securing a sufficient bonding strength between a resin case and a metal base and reducing substantially the amount of the adhesive that flows out.